JPH06308546A - Method for adjusting characteristic of optical circuit - Google Patents

Abstract

PURPOSE:To provide the method for adjusting characteristics of an optical circuit capable of efficiently controlling the local refractive index or optical path length of optical waveguides constituting the optical circuit. CONSTITUTION:A Mach-Zehnder interferometer composed of directional couplers 12a, 12b and optical waveguides 13a, 13b formed on a silicon substrate 11 is impregnated with hydrogen at room temp. and is subjected to a heat treatment, by which the sensibility with a light induced change in refractive index is easily and safely improved with good controllability. Only a part of the optical waveguide 13a not coated with a metallic film 22 is thereafter irradiated with UV light from a KrF excimer laser 23, by which the refractive index thereof is corrected and the characteristics are adjusted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for adjusting the characteristics of an optical circuit composed of an optical waveguide formed on a substrate.

[0002]

2. Description of the Related Art A quartz optical waveguide that can be formed on a quartz glass substrate or a silicon substrate has low loss and high stability.
It is known that it is very useful in constructing a practical optical circuit such as an optical multiplexing / demultiplexing circuit because of its features such as good workability and good compatibility with a silica optical fiber. Recently, making use of the above-mentioned features, the fabrication of a planar optical circuit with higher functionality and higher integration is underway.
Optical circuits that use the phase of light are highly important because they enable circuits that are difficult to achieve with electronic circuits.

In a silica-based optical waveguide, under clad layer deposition → core layer deposition → core etching → on a silicon substrate.
The core-clad structure is formed by performing the fabrication process called over-cladding layer deposition. In a highly functionalized and highly integrated optical circuit that uses the phase of light, the phase of light depends on the refractive index of the waveguide and the propagation length ( Since it depends on the optical path length, a minute refractive index in manufacturing, a change in the shape of the waveguide, and a stress applied to the waveguide have a great influence on the device characteristics. Therefore, in order to improve the productivity in a highly functionalized and highly integrated optical circuit, it is necessary to correct minute fluctuations in fabrication, etc., and it is possible to locally adjust the refractive index to adjust the characteristics of the optical circuit. Realization of the method was desired.

Taking the Mach-Zehnder (MZ) interferometer, which is an important element constituting the above-mentioned planar optical circuit, as an example, the importance of local refractive index control will be described below. The MZ interferometer is an indispensable component in constructing an optical switch and an optical demultiplexer, and an example thereof is shown in FIG.

In FIG. 2, 1a and 1b are two directional couplers, 2a and 2b are two optical waveguides connecting the directional couplers 1a and 1b, and these are formed on the silicon substrate 3. There is. Further, 4a, 4b, 4c and 4d are optical fibers connected to both ends of the two optical waveguides 2a and 2b.

In the MZ interferometer, not only the silica-based optical waveguide but also the minute refractive index, the variation of the waveguide shape, and the stress applied to the waveguide greatly affect the element characteristics. Assuming that the optical path length difference between the two optical waveguides 2a and 2b is n _f · L1, the output light intensity is f _sr = c / n _f · L1 with respect to the frequency f (or wavelength λ) of the incident light. (However, c is the speed of light) is known to have a characteristic, and using this periodic characteristic of the output light intensity, an optical device that operates as an optical switch, an optical frequency multiplexer / demultiplexer, or the like. Has been realized.

However, a manufacturing error such as a refractive index and a width of the waveguide causes a shift of the periodic characteristic of the optical interferometer due to the phase, and causes a remarkable deterioration in the operating characteristics of the optical switch and the optical frequency multiplexer / demultiplexer. It will be. Therefore, in order to improve productivity in an optical circuit using an MZ interferometer, it is necessary to compensate for minute fluctuations in manufacturing, and an effective adjustment method has been desired.

As one of the methods of compensating for the phase error in the optical interferometer, a method of utilizing a refractive index change (light-induced refractive index change) caused by irradiation with visible light or ultraviolet laser light has been reported ( For example, PTL, (3), 19
91, Hibino, et al., Pp640-642).

A similar report regarding the control of the refractive index of the optical waveguide is one regarding the change of the refractive index of the optical fiber (eg, Opt. Lett., Vol.15, 1990, B. Molo, e.
t al., pp953-955). According to this, it has been observed that in the GeO ₂ -doped silica optical fiber, the refractive index of the core is changed by 3 × 10 ⁻⁵ by ultraviolet irradiation. In addition, a similar photo-induced refractive index change Δn is also observed in a silica-based optical waveguide formed on a silicon substrate.

However, the sensitivity of the photo-induced refractive index change in the silica type optical waveguide formed on the silicon substrate was weaker than that of the optical fiber. This causes a difficulty in that a long light irradiation time is required at the time of manufacturing a photo-induced gradient index grating, which is one of the application examples, and a more stable light irradiation optical system is required. Therefore, a treatment method has been proposed for increasing the efficiency of changing the refractive index.

One is that it is possible to increase the sensitivity (efficiency) of the photo-induced refractive index change by performing a high temperature (about 500 ° C.) heat treatment in a hydrogen atmosphere (for example, Opt. Lett). ., vol.18, 1993, KDSimmons, et al.,
pp25-27) (hereinafter referred to as the high temperature hydrotreatment method). However, performing high-temperature heat treatment in a hydrogen atmosphere lacks safety, requires a special electric furnace for performing high-temperature heat treatment, and increases loss in the near infrared region by several dB or more. There was a problem (eg Electr
on Lett., (28), 1992, GDMaxwell, et al., pp2106-
See 2107).

The other is hydrogen burner and LPG.
By burning the optical waveguide with a flame such as a burner, it is possible to increase the sensitivity (efficiency) of the light-induced refractive index change (see OFC '93, KOHill, et al.) (See below, Flame sweeping method (called flame brushing).
This method is excellent in that the efficiency of changing the refractive index can be easily increased, but there is a problem in its controllability and reproducibility.

[0013]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention makes it possible to increase the sensitivity of light-induced refractive index change easily, safely and with good controllability, and to provide an optical waveguide constituting an optical circuit. An object of the present invention is to provide a method for adjusting the characteristics of an optical circuit, which makes it possible to efficiently control the local refractive index or the optical path length.

[0014]

In order to achieve the above object, the present invention provides a method for adjusting the characteristics of an optical circuit comprising an optical waveguide consisting of a core and a clad formed on a substrate. We propose a method for adjusting the characteristics of an optical circuit, which comprises a step of impregnating hydrogen at room temperature, a step of heat-treating the optical waveguide, and a step of locally irradiating the optical waveguide with visible light or ultraviolet light.

[0015]

According to the present invention, since the step of impregnating hydrogen and the step of performing heat treatment are separated, the step of impregnating hydrogen can be performed at room temperature, which is more efficient than the high temperature hydrotreatment method. Since the safety is high and the heat treatment is performed in the atmosphere or He atmosphere, the heat treatment can be performed easily by using an ordinary electric furnace or the like. Further, the sensitivity (efficiency) of the change in refractive index can be controlled and reproducible by the hydrogen pressure in the step of impregnating hydrogen and the time for placing the waveguide in a hydrogen atmosphere, and the heat treatment temperature and heat treatment time in the heat treatment step. be able to.

[0016]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention, in which a sensitivity-improving treatment for a photo-induced refractive index change is carried out, and then a M-shaped excimer laser is used in a silica optical waveguide.
An example in which the characteristics of the Z interferometer are adjusted is shown.

In FIG. 2, 10 is an MZ interferometer,
Two directional couplers 1 formed on a silicon substrate 11
2a, 12b and two optical waveguides 13a connecting them,
It consists of 13b. In addition, 21a, 21b, 21
c and 21d are optical fibers connected to both ends of the two optical waveguides 13a and 13b, 22 is a shielding metal film, and 23 is K.
rF excimer laser, 24 is a mirror, 25 is a lens, 2
Reference numeral 6 is a cylindrical lens.

The MZ interferometer 10 is an asymmetric type, and the optical waveguide 13a is longer than 13b. This type of MZ interferometer operates as a frequency or wavelength demultiplexer.
Here, the optical path length difference Δ between the two optical waveguides 13a and 13b
L ₂ was set to about 1 cm, and the incident light was designed to be demultiplexed at intervals of 10 GHz.

The method of the present invention will be described below.

First, a GeO ₂ -doped silica glass waveguide type MZ interferometer 10 was produced on a silicon substrate 11 by a usual method. Here, the core of the waveguide was rectangular and the size was 7 × 7 μm. Further, the refractive index difference between the core and the clad was set to 0.75%. In the directional couplers 12a and 12b, the coupling rate is set to about 50% at a wavelength of 1.3 μm.

The MZ interferometer 10 prepared above was placed in a H ₂ gas high-pressure enclosure as shown in FIG. 3, the hydrogen gas pressure was set to 5 atm, and hydrogen was impregnated for about 24 hours. In FIG. 3, 27 is a container, 28 is a hydrogen gas supply system, 29 is a pressure gauge, and 30 is a leak valve.
Subsequently, the MZ interferometer 10 impregnated with hydrogen was annealed at 200 ° C. for about 24 hours in an electric furnace to perform a treatment for improving the sensitivity of the photo-induced refractive index change.

The waveguide type MZ interferometer 10 which has been subjected to the above-mentioned processing is subjected to wavelength conversion from the KrF excimer laser 23 through the mirror 24, the lens 25 and the cylindrical lens 26.
Irradiation with an ultraviolet laser beam of 248 nm was performed and the change in characteristics before and after irradiation was examined. The laser light was applied from above the MZ interferometer 10 to the portion not covered with the metal film 22. Therefore, the laser light induces a refractive index change only in a part of the optical waveguide 13a. At this time, the KrF excimer laser 23
The irradiation power was 100 mJ / cm ² · pulse, the pulse repetition frequency was 5 Hz, and the irradiation time was 2 minutes.

In order to investigate the characteristics of the MZ interferometer 10 described above, a current-swept semiconductor laser having a center wavelength of 1.3 μm was introduced from the optical fiber 21a. In this example, the characteristic change of the MZ interferometer 10 could be monitored during the irradiation of the laser beam.

FIG. 4 shows the optical fiber 2 after irradiation for 2 minutes.
The wavelength dependence of the output from 1c is shown in comparison with that before irradiation. As shown in FIG. 4, before irradiation, the optical frequency f _i
The characteristic of the MZ interferometer 10 which was extinguished at 1. could be adjusted so that it would be extinguished at a desired optical frequency f ₀ after irradiation. The adjustment from the optical frequency f _i to f ₀ is based on the photoinduced refractive index change Δn due to the irradiation of the KrF excimer laser light.

In the conventional optical waveguide, the optical frequencies f _i to f
To obtain the refractive index change Δn corresponding to the change to ₀ ,
Irradiation power of KrF excimer laser 100 mJ / cm ² ·
pulse, pulse repetition frequency 50Hz, irradiation time about 2
It took 0 minutes. Therefore, according to the present invention, it was confirmed that the sensitivity (efficiency) of light-induced refractive index change was improved about 100 times.

The hydrogen gas used for the sensitivity-improving treatment for the photo-induced refractive index change may be any gas that impregnates quartz glass and causes a reduction reaction. Also,
In the present invention, it is also possible to partially adhere a metal such as amorphous silicon or stainless steel to the optical waveguide and impregnate hydrogen only in a part of the optical circuit to selectively improve the sensitivity of the light-induced refractive index change. is there. In addition, the sensitivity of the light-induced refractive index change can be similarly improved by bringing a silicone resin or the like into close contact with the optical waveguide and performing heat treatment.

Since the change in the refractive index of the core is caused by the absorption associated with GeO ₂ having a wavelength of 240 nm, the laser used for the change in the refractive index may be any laser having an oscillation wavelength near 240 nm. Also, a laser having an oscillation wavelength in the visible region can be used. This is because even in a visible region laser, a similar change is induced by two-photon absorption. In summary, the laser used in the present invention is a He-Cd laser, N
₂ lasers, various excimer lasers, Ar ion lasers, N
d ³⁺ : YAG laser, alexandrite (Cr ³⁺ : B
eAl ₂ O ₄ ) laser second, third, fourth harmonics, etc.
Any material having a wavelength in the ultraviolet / visible region may be used.

The GeO ₂ concentration is not particularly limited because it can be adjusted by the length of the MZ interferometer. Further, in the above embodiment, G is used as a dopant.
eO ₂ was used, but in addition to it, TiO ₂ , Ce ₂ O _3, etc.
A dopant having absorption in the ultraviolet region and sensitive to ultraviolet rays may be used. The present invention also relates to the optical fibers 21a to 21.
It does not prevent the use of an optical fiber having a polarization maintaining property for d.

FIG. 5 shows a second embodiment of the present invention. Here, an example is shown in which a photo-induced grating is produced in a silica-based optical waveguide after a sensitivity improving process for a photo-induced refractive index change is performed. Indicates.

In FIG. 5, 40 is a silica-based optical waveguide, 4
1 is an alexandrite (Cr ³⁺ : BeAl ₂ O ₄ ) laser, 42a and 42b are wavelength conversion elements (BBO), 43
Is a wavelength selection mirror, 44a, 44b, 44c and 44d are mirrors, 45 is a lens, 46 is a cylindrical lens,
47 is a half mirror.

The method of the present invention will be described below.

First, Ge is formed on a silicon substrate by a usual method.
An O ₂ -doped silica glass waveguide 40 was produced. here,
The core of the waveguide was rectangular and the size was 7 × 7 μm.
Further, the refractive index difference between the core and the clad was set to 0.75%.

The produced silica-based optical waveguide 40 is shown in FIG.
The sample was placed in the H ₂ gas high-pressure container shown in (4), the hydrogen gas pressure was set to 5 atm, and hydrogen was impregnated for about 72 hours. Then, the optical waveguide 40 impregnated with hydrogen is placed in an electric furnace.
Annealing was performed at 200 ° C. for about 72 hours to improve the sensitivity of the photo-induced refractive index change.

The alexandrite laser 41 is applied to the optical waveguide 40 which has been subjected to the above-mentioned processing, and the wavelength conversion element 42a,
42b, wavelength selection mirror 43, mirrors 44a to 44d,
The third harmonic wave (3ω) was irradiated from above through the lens 45, the cylindrical lens 46, and the half mirror 47 for about 8 minutes to fabricate a photoinduced grating. On this occasion,
The laser light intensity is about 400 mJ / cm ² · pulse, the pulse repetition frequency is 20 Hz, and the length of the irradiated optical waveguide is 1
It was set to 0 mm.

In order to examine the characteristics of the grating, light from an LED having a center wavelength of 1.55 μm was introduced into an optical waveguide by a fiber and measured using a spectrum analyzer.

FIG. 6 shows the reflection spectrum characteristics of the optical waveguide manufactured according to the present invention in comparison with the ordinary optical waveguide.

In order to fabricate a photo-induced grating on a normal silica-based optical waveguide under the same light irradiation conditions, several tens are required.
Although a light irradiation time of more than one minute is required, it was difficult to keep the light irradiation optical system as shown in FIG. 5 stable during that time, so that sufficient characteristics could not be obtained. By performing the sensitivity improvement process of the induced refractive index change,
It was possible to fabricate a photoinduced grating exhibiting a sufficient reflectance, here about 85%.

[0039]

As described above, according to the present invention, since the step of impregnating hydrogen and the step of performing heat treatment are separated, the step of impregnating hydrogen can be performed at room temperature, and high temperature hydrogenation can be performed. Compared with the treatment method, it is more safe, and since the heat treatment is performed in the atmosphere or He atmosphere, it is possible to use an ordinary electric furnace or the like, and the heat treatment can be easily performed. Further, the sensitivity (efficiency) of the change in refractive index can be controlled and reproducible by the hydrogen pressure in the step of impregnating hydrogen and the time for placing the waveguide in a hydrogen atmosphere, and the heat treatment temperature and heat treatment time in the heat treatment step. It is possible to do without changing the conventional method of manufacturing an optical circuit,
It is possible to effectively and simply provide an optical circuit whose characteristics are adjusted. Further, according to the present invention, since an optical circuit that has already been manufactured can be implemented as a target, a circuit having a nonstandard output characteristic can have desired characteristics, and the productivity of the optical circuit improves. Further, according to the present invention, a large refractive index change Δn can be obtained by irradiating light for a short time, so that the characteristics of the photo-induced grating can be improved.

[Brief description of drawings]

FIG. 1 is a layout diagram of an optical system showing a first embodiment of the present invention.

FIG. 2 is a configuration diagram showing an example of an MZ interferometer.

FIG. 3 is a diagram showing an example of a hydrogen gas high-pressure enclosure.

FIG. 4 is a diagram showing output light wavelength spectrum characteristics in the first embodiment.

FIG. 5 is a layout diagram of an optical system showing a second embodiment of the present invention.

FIG. 6 is a diagram showing reflection spectrum characteristics in the second embodiment.

[Explanation of symbols]

10 ... MZ interferometer, 11 ... Silicon substrate, 12a, 12
b ... Directional coupler, 13a, 13b ... Optical waveguide, 21a
21d ... Optical fiber, 22 ... Metal film, 23 ... KrF excimer laser, 24, 44a-44d ... Mirror, 25,
45 ... Lens, 26, 46 ... Cylindrical lens, 4
0 ... Quartz-based optical waveguide, 41 ... Alexandrite laser, 42a, 42b ... Wavelength conversion element, 43 ... Wavelength selection mirror, 47 ... Half mirror.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Masao Kawauchi 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation (72) Inventor Hiroro Yamada 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. A method for adjusting characteristics of an optical circuit including an optical waveguide including a core and a clad formed on a substrate, the method including impregnating the optical waveguide with hydrogen at room temperature, and heat treating the optical waveguide. And a step of locally irradiating the optical waveguide with visible light or ultraviolet light, the method for adjusting characteristics of an optical circuit.